/Engine/Source/Runtime/Engine/Classes/GameFramework/GameModeBase.h
Allows game to send network messages to provide more information to the client joining the game via NMT_GameSpecific (for example required DLC) the out string RedirectURL is built in and send automatically if only a simple URL is needed
virtual void GameWelcomePlayer ( [UNetConnection](API\Runtime\Engine\Engine\UNetConnection) * Connection, [FString](API\Runtime\Core\Containers\FString) & RedirectURL )